For the video sequences captured by a relative static camera, the video contents often consist of some background areas. For example, the surveillance and teleconferencing sequences often exhibit this type of video. The background information can be utilized for video coding to reduce the required bitrate for the video sequence. For example, background prediction is a technique developed to take advantage of similar background data in the reference pictures. The technique utilizes prediction based on global static background to achieve more efficient prediction. Since the generated or selected background picture is static and doesn't need to update or only needs to update infrequently, the background picture can be generated and encoded in high quality without consuming high bitrates. Then the high-quality background picture can provide a high-quality reference for the subsequent pictures, which have similar background areas in the background picture.
One example of video coding standard that adopts the background prediction is AVS (audio and video coding standard) developed by Audio and Video Coding Standard Workgroup of China. The background prediction is used in the surveillance profile of AVS and a short summary of the surveillance profile of AVS is described as follows.
An I-picture can be marked as a G-picture and stored in the reference picture buffer to provide a global background reference for the subsequent frames. With the help of G-picture, the following P-pictures can choose to use G-picture as the last reference in the reference picture lists, or another picture, i.e., S-picture can select the G-picture as its only reference picture with zero motion vectors.
At the encoder side of an AVS based system, the G-picture can either be selected from original input frames or generated from a group of input frames. At both encoder and decoder sides, the reconstructed G-picture is stored in the G-picture reference buffer. In the encoding and decoding process for each current picture, the G-picture can be selected to replace the last reference picture in the reference picture lists.
In this disclosure, a matched block for a current block is defined as the block in the background picture that results in small absolute differences between the matched block and the current block. The corresponding block of the current block is defined as the block in the background picture that is located at the same picture location. The reference block of the current block is defined as the block in the reference picture that is pointed to by the motion vector associated with the current block.
The AVS system as described above uses background reference prediction, which works effectively for the so-called “pure background blocks” having fully matched corresponding block in the high-quality background picture. However, for the so-called “pure foreground blocks”, there is hardly any matched corresponding block in the background picture. Therefore, background reference prediction will not help to improve coding efficiency for foreground blocks and should not be selected.
In the AVS system, for a “hybrid-foreground-and-background block”, only a partial block is matched with its corresponding block in the background picture. Therefore, background reference prediction is not very effective either since unmatched part cannot be predicted efficiently. The “hybrid-foreground-and-background blocks” usually exist in the boundary areas between foreground areas and background areas.
It is desirable to develop techniques that can also work efficiently for the hybrid-foreground-and-background blocks.